A mixer is an essential building block in a radio frequency (RF) receiver for translating in frequency a modulated signal from being centered about the RF carrier or an intermediate frequency (IF) to being centered about DC, where it is referred to as the baseband (BB) signal. In the remainder of this document references to RF frequencies or RF signals equally apply to mixers whose inputs are signals at IF frequencies. The elements that actually perform the frequency translation are commutating switches (mixer core) that direct an RF current alternately to the opposite sides of the load impedance. Mathematically the differential voltage across the load is equal to the RF current multiplied by the differential load impedance and an alternating sequence of 1s and −1s. The said sequence is the effective commutating function that in an ideal, balanced configuration should have 50% duty cycle. As the RF input is often a voltage, transistors are mostly used to convert the said RF voltage into current before the commutating switches. The combination of the transistors performing the voltage-to-current (V-I) conversion (transconductor), commutating switches and load impedance, as shown in FIG. 1, are commonly known as an active mixer. A single-transistor transconductor 1 followed by one pair 2 of differential or balanced switching transistors, as shown in FIG. 1a, is referred to as a single balanced mixer. A differential or balanced transconductor 3 followed by two pairs 4, 5 of balanced switching transistors, as shown in FIG. 1b, is referred to as a double balanced mixer. Constructed with active devices, both the transconductor 1, 3 and the commutating switches 2, 4, 5 may have nonlinear signal transfer characteristics of both even and odd order.
In many applications large interfering, radio frequency, signals called blocking signals are present at the input of the mixer together with the desired input signal. Although there is usually a reasonably large frequency separation between the desired and blocking signals, passive RF filters prior to the mixer can only attenuate the blocking signals to a limited extent. Residual blocking signals reaching the mixer input may be translated to the baseband by even-order nonlinearity in the demodulator. The phenomenon is loosely referred to as envelope detection because any amplitude modulation present in the blocking signal will be converted into a varying signal in the baseband, in addition to the DC component representing the average power of the undesired BB signal. Even-order nonlinearity in the RF receiver in general and mixers in particular can therefore adversely affect the detection of the desired signal in zero-IF (direct conversion) and low-IF receiver architectures because the said desired signal is directly frequency-shifted to the baseband before sufficient amplification. A commonly used figure of merit for describing linearity in terms of second order distortion is known as second order intercept point or IP2. Similarly, higher even order distortion can be described by IP4 for fourth order, IP6 for sixth order, etc, intercept points. Low-IF or direct conversion architectures require mixers with high even-order intercept points.
In a fully differential or balanced mixer implementation, the blocking signals envelope-detected by even-order nonlinearity should ideally be equal at both the positive and the negative output nodes so that the differential output is zero, leaving the desired signal unaffected. Inevitable mismatch of a practical implementation of the positive and negative signal paths of the mixer, however, results in imperfect cancellation of the envelope-detected blocking signal. Well-matched differential circuits are therefore also considered to have high IP2. Since RF devices tend to be small in order to achieve high frequency operation, matching accuracy among them is limited. Typical achievable IP2 by a fully integrated mixer is 40˜50 dBm, which is insufficient for advanced applications such as WCDMA, in which a mobile phone's transmitter signal leaks through the duplexer into the phone's own receiver, where it acts as a blocking signal. Without an expensive SAW filter after the low noise amplifier (LNA), the receiver's mixers would require an IP2 of the order of 75 dBm in a direct-conversion architecture. Such a requirement is 1000 times higher than the state-of-the-art.
Reference is now made to the following documents:    [1] K. Kivekäs, A. Pärssinen and K. Halonen, “Characterization of IIP2 and DC-Offsets in Transconductance Mixers”, IEEE Trans. Circuits and Systems, Vol. 48, No. 11, pp. 1028-1038, November 2001    [2] D. Manstretta et al, “Second-Order Intermodulation Mechanisms in CMOS Downcoverters”, IEEE J. Solid-State Circuits, Vol. 38, No. 3, pp. 394-406, March 2003    [3] Jussi Ryynänen et al, “A Single-Chip Multimode Receiver for GSM900, DCS 1800, PCS 1900 and WCDMA”, IEEE J. Solid-State Circuits, Vol. 38, No. 4, pp. 594-602, April 2003
Both references [1] and [2] identify many sources of nonlinearity in the mixer, which must all be properly addressed if any method is to improve the overall IP2 significantly. Recognizing the influence of circuit mismatch on envelope detection, reference [3] suggested trimming the load impedance at the mixer output during power-up as a way to improve IP2. U.S. Pat. No. 6,393,260 B1 discloses a trimming method to improve mixer balance by empirical bias adjustment based on repeated measurements. Perfect balance is however not generally possible for a double balanced mixer without separately adjusting the pair of transconductor transistors and each pair of switching transistors. Performed outside the normal operation of the mixer, the method also requires memory elements, A/D and D/A converters and preferably an RF test signal source, which adds a large cost overhead. The requirement of an RF test signal makes the method mostly suitable for production testing only and the non-volatile memory needed to store the final settings requires special integration technology.